Arrhythmias and other signs of cardiac dysfunction occasionally follow electric cardioversion. In order to yield insight into subcellular alterations which might be responsible for this dysfunction, high-intensity electric shocks (comparable in voltage gradient at the cell level to those used clinically)will be given to myocardial cells from chick embryos which are cultured with techniques facilitating cell differentiation; the resulting changes in ultrastructure will be correlated with changes in photocell mechanograms. The mechanisms responsible for the structural and contractile alterations observed will be studied by examining these changes as well as changes in intra-cellular concentration and localization of calcium, sodium, and magnesium ions using electron microscopic histochemical techniques, when the shock is given under varying environmental conditions. BIBLIOGRAPHIC REFERENCES: Lepeschkin, J.L. Jones, and T. C. Gibson. Analysis of ventricular fibrillation after atrial cardioversion. 57th Ann. Session, American Coll. Physicians, Phila, PA, April 5-8, l976. J.L. Jones, C. Proskauer, R.E. Jones, W.K. Paull, E. Williams, and E. Lepeschkin. Morphological and Electrophysiological maturation of chick myocardial cells in monolayer (cell) culture. Submitted for Publication Feb. 23, l976.